“Man overboard!” is an exclamation given aboard a vessel to indicate that someone (not necessarily a “man” but historically one of the crewmen “manning” the vessel) has fallen off of the ship into the water and is in need of immediate rescue.
Current man overboard technology consists of three major types of beacons:                1. Automatic Identification System (AIS), an automatic tracking system used on ships and by vessel traffic services (VTS).        2. EPIRB (Emergency Position Indicator Radio Beacon, which are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress.        3. A PLB (personal locator beacon) is a particular type of EPIRB that is typically smaller, has a shorter battery life and unlike a proper EPIRB is registered to a person rather than a vessel.        
AIS and PLB beacons are focused towards the individual whereas EPIRB beacons are dedicated to the vessel. While all three types share similarities, AIS beacons are focused on rescue by the mother ship and often automatically activated. Contrarily PLB and EPIRB beacons are manually activated and do not alarm the mother ship but rather the coast guard.